The present invention relates to a metal halide lamp having a ceramic discharge tube.
A conventional metal halide lamp generally comprises a discharge tube having a pair of electrodes therein, an outer tube containing the discharge tube in the exhausted state or in a state being filled with an inert gas such as N.sub.2 -gas, stem wires leading out of the discharge tube in order to supply a current to the pair of electrodes in the discharge tube from a socket of an external lighting apparatus, and a current supply member configured in a base or the similar member and connected with ends of the stem wires outside the discharge tube.
The discharge tube is made of quartz glass having a transparent or translucent property. Further, in the conventional metal halide lamp, it is known that a ceramic material having a transparent or translucent property is used for the discharge tube as disclosed in unexamined and published Japanese patent application TOKKAI (Hei) No. 6-196131, for example.
A metal halide as a luminescent material, a rare gas such as Ar-gas for a start of a lighting operation and mercury are filled into the discharge tube. As concrete examples of the metal halide, there are sodium iodide, thallium iodide, and dysprosium iodide or the like. The conventional metal halide lamp emits a light having emission spectrum of visible region by combining a plurality of the above-mentioned metal halide. The conventional metal halide lamp is connected to an electric power source via a stabilizer. Accordingly, the current is limited so as not to exceed a predetermined value during the light operation.
Processes of the lighting operation in the conventional metal halide lamp are described as follows.
Firstly, a discharge is started caused by dielectric breakdown in both the rare gas and a vapor of the mercury, and thereby, temperatures on the inside walls of the discharge tube rise. According to the temperature rise, the metal halide filled into the discharge tube are vaporized. According to this vaporization of the metal halide, the light is radiated outside the discharge tube as light output with emission spectrum defined by vaporized metal atoms.
In the above-mentioned conventional metal halide lamp, at least one of the following methods (1) and (2) are selected and used in order to start and restart the lighting operation easily.
(1) The method of applying a pulse voltage for a starting operation to the pair of the electrodes by means of an igniter disposed outside the discharge tube.
(2) The method of coating the pair of the electrodes with an emitter material having a property of emission of electron and made of metal oxide compound such as barium oxide and scandium oxide.
However, in the case that the method (1) is used in the conventional metal halide lamp, it is strongly required that the pulse voltage is reduced as low as possible from the aspect of safety.
On the other hand, in the case that the method (2) is used in the conventional metal halide lamp, there occurs a problem that the luminescent material to be filled into the discharge tube is limited by the emitter material. Especially, it is impossible that rare earth metal compound such as dysprosium iodide is filled into the discharge tube as the luminescent material because the rare earth metal makes a chemical reaction with the emitter material.
Furthermore, in the conventional metal halide lamp, as a measure for reducing the above-mentioned pulse voltage, it has been suggested that a conductor made of molybdenum is disposed adjacent to the discharge tube as a proximity conductor. That is, in this suggestion, in order to start and restart the lighting operation easily, a predetermined electric potential is given to the proximity conductor from the electric power source, so as to prompt the dielectric breakdown in both the rare gas and the vapor of the mercury inside the discharge tube.
However, in the case that the proximity conductor is used in the conventional metal halide lamp, there is a possibility that the proximity conductor gives undesirable influence on the luminescent material in the discharge tube and therefore, the conventional metal halide lamp equipped with the proximity conductor can not be applied for practical use. The undesirable influence is that when the proximity conductor is used in the conventional metal halide lamp, a photoelectron is radiated from the proximity conductor owing to a large energy of rays including an ultraviolet ray. Thereby, an outer surface of the discharge tube is covered with the photoelectron, so that alkaline metal such as sodium leaks from the inside of the discharge tube to the outside thereof (i.e., to the inner space of the outer tube) through walls of the quartz glass. As a result, there occurs change of a color in the light output. Furthermore, a lamp voltage is increased during the lighting operation, and thereby, there is a problem that the conventional metal halide lamp can not be lit. With respect to the problem that the alkaline metal in the discharge tube leaks to the inner space of the outer tube, it appears regardless of the state in the outer tube. That is, the problem occurs not only in the exhausted state remarkably but also in the state being filled with N.sub.2 -gas.